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The catalytic converter has an oxygen sensor in front and behind it. When the vehicle is warm and running in closed loop mode, the upstream oxygen sensor waveform reading should fluctuate. The downstream O2 sensor reading should be fairly steady. Typically the P0420 code triggers the Check Engine Light if the readings of the two sensors are similar. This is indicative of (among other things) a converter that is not working as efficiently as it should be (according to specs). It is part of the vehicle emissions system.

A code P0420 may mean that one or more of the following has happened:

* Leaded fuel was used where unleaded was called for * An oxygen sensor is not reading (functioning) properly * The engine coolant temperature sensor is not working properly * Damaged or leaking exhaust manifold / catalytic converter / exhaust pipe * Retarded spark timing * The oxygen sensors in front and behind the converter are reporting too similar of readings

To Do list

* Check for exhaust leaks at the manifold, pipes, catalytic converter. Repair as required. * Use a scope to diagnose the oxygen sensor operation (Tip: The oxygen sensor in front of the catalytic converter normally has a fluctuating waveform. The waveform of the sensor behind the converter should be more steady). * Inspect the downstream heated oxygen sensor (HO2), replace if necessary * Replace the catalytic converter

The turbine shaft speed sensor can cause this problem, It is also know as the transmission input shaft speed sensor. It will cause the transmission to **** and also stick in 3rd gear, which wound cause slow acceleration.

"Fuel system too lean at idle". It's an air/fuel ratio that is not correct, possibly a vacuum leak somewhere. Or a leaky exhaust pipe. Could be related to intake manifold gasket. Somewhere, air is getting into the engine.

I suggest taking it back to the dealer under warranty. I think Hyundai is 10yr./100k for the original owner. 5yr./50k if not. Ask first if covered, and ask why not id not covered.

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1. The thermostat could be stuck closed creating more pressure than normal. Your temp guage would show a higher temperature or you have another blockage in the system.
2. The expansion tank is too full.
3. The cylinder head may be leaking. If you fill the system run the engine to normal temperature with the radiator cap off, look for quite large bubbles coming out of the filler cap. If the head is badly leaking, they can be like a volcano. The temp will rise quickly from cold and the engine will overheat in a short distance.
It does seem odd that it would only spill over after 20 mins. Suspect that the expansion is over full first if your temp is normal. Let it run until it stops and see where the level is when the engine is cold.